Cementitious compositions and their use in corrosion protection

ABSTRACT

A cementitious composition ( 1 ) which, in the hardened state, acts as a low leaching coating in low alkalinity and low hardness waters. The composition comprises at least one cement and a highly reactive pozzolan. The cementitious composition can be applied to a surface such as a metallic drinking water pipe ( 2 ) in order to provide corrosion protection therefor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to cementitious compositions, and relatesto the use of such compositions in corrosion protection.

2. The Prior Art

The present invention relates to cementitious compositions, and relatesto the use of such compositions in corrosion protection.

The corrosion of metals in contact with water occurs by an aqueouselectrochemical mechanism. This involves the presence of watercontaining dissolved ions at the metal surface, and the correspondingtransfer of the electrons from the metal surface to the aqueousenvironment in contact with it. Corrosion protection involvesinterfering with these processes.

Current internal protection systems for water pipes usually consist ofeither cement mortars or epoxy resin polymeric coatings. Cement mortarssuffer from a number of problems, such as lime leaching when placed inlow alkalinity and low hardness waters. This results in impaired waterquality by increasing the pH (ie the alkalinity) and calciumconcentrations, and causes structural deterioration of the cementmortar, which results in poor durability. Epoxy resins can also sufferfrom a number of problems: if they are insufficiently cured, then theremay be problems with water quality; there may be blistering; and theremay be poor coverage at pipe joints.

SUMMARY OF THE INVENTION

We have now found a way to improve corrosion protection of metalsurfaces, such as metallic drinking water pipes. We achieve this byproviding a cementitious composition which, in the hardened state, actsas a low leaching and durable coating in low alkalinity and low hardnesswaters.

According to one aspect of the invention there is provided a method ofprotecting a surface from corrosion, comprising applying to the surfacea cementitious composition which, in the hardened state, acts as a lowleaching coating in low alkalinity and low hardness waters.

The method can be used to protect a wide variety of surfaces fromcorrosion. The method is of particular use in preventing the corrosionof metal pipes from corrosion caused by moisture. The method hasparticular application to the protection of the inside of drinking waterpipes.

After the cementitious composition has been applied to the surface, itcan be left to harden. In general, sufficient hardening will occurwithin about 24 hours, but it may take several weeks for the compositionto finish curing.

The thickness of the composition depends upon the application. Typicallythe composition would be applied to a thickness in the range 2 to 16 mm,and usually the thickness would be in the range 4 to 8 mm.

The cementitious composition advantageously comprises at least onecement and a component which is capable of reacting with free lime. Thisreduces the amount of lime available to leach. Advantageously, thecementitious composition comprises at least one hydraulic cement and ahighly reactive pozzolan—the pozzolan is capable of reacting with thefree lime.

It is preferred that the composition comprises not more than 50 parts byweight pozzolan, based on the weight of the dry composition, and notless than 5 parts by weight pozzolan based on weight of the drycomposition. It is more preferred that the composition comprises notmore than 30 parts by weight pozzolan, based on the weight of the drycomposition.

In one embodiment, the cementitious composition may further comprise acement replacement material, preferably in an amount of not more than 70parts by weight, based on the dry composition. The cement replacementmaterial assists in producing a dense cement with a fine pore structurein the hardened cement paste. The cement replacement material ispreferably ground granulated blast furnace slag (GGBFS—available fromCivil and Marine Slag Cement Limited), activated blast furnace slag,pulverised fuel ash (PFA—available from Pozzolanic Lytag) and/ormicrosilica powder (e.g. grade 940U available from Elkem Materials).

In certain embodiments, the composition may comprise not more than 10parts by weight pozzolan, based on the dry composition. For example,when the cement replacement material is provided, it is preferred thatthe composition comprises not more than 10 parts by weight pozzolan,based on the dry composition.

A wide variety of cements may be used. Ordinary Portland cement (OPC)has been found to be very suitable. Examples of three commerciallyavailable cements that are suitable for use with the present inventionare: ordinary Portland cement to B.S. 12:1991 class 42.5N; Ferrocretewhich is a rapid-hardening Portland cement to B.S. 12:1991 class 52.5N;and Sulfacrete which is a sulphate resisting Portland cement to B.S.4027:1991 class 42.5N. All three of these cements are available fromBlue Circle Industries.

The cement may be a Microcem cement, i.e., a superfine Portland cementhaving a greater surface area than conventional Portland cements.Microcem 550, Microcem 650SR or Microcem 900 may, for example, be usedin the invention.

In an embodiment, the cement is a hydraulic cement. The hydraulic cementis preferably calcium silicate cement.

Pozzolans are silaceous and aluminous materials, such as certain flyashes and blast furnace slags, which, in finely divided form, willexhibit cementitious properties when mixed with, for example, lime andwater. We prefer to use a highly reactive pozzolan. We have obtainedexceptionally good results when the pozzolan is metakaolin. Metakaolinscan be formed from Kaolinite (Al₂Si₂O₅(OH)₄) by heating, for example toabout 500° C. to about 800° C. Metakaolin is available from, forexample, English China Clay International. Grade Metastar 501, availablefrom English China Clay International, has been found to be verysuitable.

The cementitious composition used in the method according to theinvention may comprise other materials, such as aggregate and areinforcement material such as a fibre reinforcement. The aggregatewould typically be a fine grade silver sand having a moisture contentof, for example, 4.7%. The reinforcement may be a polymeric fibre andmay be present in an amount up to 40 parts by weight, based on the totalweight of the dry composition.

The cementitious material may include other materials, such as silicafume.

In general, the cementitious composition according to the invention ismixed with water before use. This results in the formation of a cementpaste. We prefer that the composition comprises not more than 50 partsby weight water, based on the wet composition.

The composition may be mixed with sand in order to form a cementitiousmortar. This cementitious mortar may then be applied to the surface tobe protected. In general, the amount of sand will be about the same asthe amount of cement.

In accordance with the invention a particularly preferred cementitiouscomposition comprises:

-   -   Metakaolin: 5 to 15 wt %    -   OPC: 10 to 30 wt %    -   GGBFS: 60 to 80 wt %

The most preferred cementitious composition comprises:

-   -   Metakaolin: 10 wt %    -   OPC: 20 wt %    -   GGBFS: 70 wt %

A particularly effective cement mortar composition would comprise 1 partby weight of the cement composition with 1 part by weight sand and0.35–0.55, preferably 0.40 to 0.45 parts by weight water.

According to another aspect of the invention there is provided acementitious composition which, in the hardened state, acts as a lowleaching coating in low alkalinity and low hardness waters.

Advantageously, the cementitious composition comprises at least onecement and a component that is capable of reacting with free lime, whichcomponent is preferably a highly reactive pozzolan.

Preferably, the composition comprises not more than 30 parts by weightpozzolan, based on the dry composition, and the composition comprisesnot less than 5 parts by weight pozzolan, based on the dry composition.

The cementitious composition according to this aspect of the inventionmay be provided with any combination of the features of the cementitiouscomposition described in relation to the method of protecting a surfacefrom corrosion protection. The cementitious composition may be mixedwith sand to form a cementitious mortar composition.

The method and composition according to the present invention make itpossible to protect pipes, especially drinking water pipes, fromcorrosion, without the usual problems associated with cement mortars orepoxy resins.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a flat surface that has been treatedwith a cementitious composition according to the invention;

FIG. 2 is a cross-sectional view of a pipe that has been treated with acementitious composition according to the invention;

FIG. 3 is a graph of pH of contact water versus age for compositionsmade in accordance with Example 1; and

FIGS. 4–6 are graphs of pH of contact water versus age for compositionsmade in accordance with Example 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 a substantially planar metallic substrate 10 has been coatedwith a layer 12 of a cementitious composition comprising a hydrauliccement, a pozzolan and water. The cementitious composition was appliedin the form of a paste to form the layer 12 and was then left to set.The cementitious material cured while in contact with the metallicsubstrate 10 and provides an alkaline environment in contact with thesurface of the substrate 10. Once fully cured, the cementitious coatingprotects the substrate 10 from electrochemical corrosion and also fromleaching of the cement by drinking water.

In FIG. 2 a substantially cylindrical metallic pipe 14 has been coatedwith a layer 16 of a cementitious composition comprising a hydrauliccement, a pozzolan and water. As in FIG. 1, the cementitious compositionwas applied in the form of a paste to form the layer 16 and was thenleft to set. The cementitious material cured while in contact with theinternal surface of the pipe 14 and provides an alkaline environment incontact with the internal surface of the pipe 14. Once fully cured, thecementitious coating product protects the pipe 14 from electrochemicalcorrosion and also from leaching of the cement by drinking water.

The following examples illustrate the invention.

EXAMPLE 1

Four cement compositions were prepared. One composition comprises 100 wt% OPC. The other three compositions comprised: 90 wt % OPC, 10 wt %metakaolin; 80 wt % OPC, 20 wt % metakaolin; and 70 wt % OPC, 30 wt %metakaolin.

Each composition was mixed with water and sand at the sand:cement:waterratio 1:1:0.45.

Blocks of the cement were exposed to a low alkalinity water and the pHwas monitored with time. There was a 24 hour retention time. The waterhardness was 20 mg/l as CACO₃. The results are shown in FIG. 3, fromwhich it is clear that the compositions containing metakaolin performedmuch better than the other compositions.

EXAMPLE 2

A cement composition was made comprising:

-   -   (1) 20 wt % OPC to B.S. 12:1991 class 42.5N available from Blue        Circle Industries.    -   (2) 70 wt % GGBFS from Civil and Marine Slag Limited    -   (3) 10 wt % metakaolin, grade Metastar 501 from English China        Clay International (which is a particularly highly purified form        of metakaolin).

Two ductile iron pipes of 100 mm diameter and 3 m total length were cutinto 12×250 mm length sections. Each pipe was then in situ lined usingstandard small bore pipe rehabilitation equipment with the assistance ofa local contractor. One pipe was lined with OPC blended with sand andwater in the ratio 1:1:0.4. The second pipe was lined with the ternarycomposition according to the invention blended with sand and water inthe ratio 1:1:0.43. The difference in water quantity was to produceoptimum pump and surface finish properties.

After lining, the pipe sections were placed in airtight bags (toeliminate air circulation) with an environment humidity of about 62%,and were allowed to cure overnight at about 10° C. for about 24 hours.

The effectiveness of the ternary blend was compared with the OPC overthree different water hardnesses, and the results are shown in FIGS. 4to 6. In each case the retention time was 8 hours. In FIG. 4 there was10 mg/l as CaCO₃, in FIG. 5 there was 35 mg/l as CaCO₃ and in FIG. 6there was 55 mg/l as CaCO₃. In each case the composition according tothe invention resulted in significant reductions in lime leaching.

It will be appreciated that the invention may be modified within thescope of the appended claims.

1. A method for protecting a metal drinking water pipe used fordelivering drinking water from corrosion due to water passingtherethrough comprising the steps of: (a) providing a metal drinkingwater pipe, (b) depositing a cementitious composition on an insidesurface of said metal drinking water pipe to form a lining therein, saidcementitious composition comprising portland cement and 5–30 weight %metakaolin, based on the weight of the cementitious composition whendry, and (c) hardening said cementitious composition.
 2. A methodaccording to claim 1, wherein the cementitious composition comprises notmore than 10 parts by weight metakaolin.
 3. A method according to claim1, wherein part of the portland cement is replaced with a cementreplacement material in an amount of not more than 70 parts by weight,based on the weight of the cementitious composition when dry.
 4. Amethod according to claim 3, wherein said cement replacement material isat least one of ground granulated blast furnace slag and pulverized fuelash.
 5. A method according to claim 1, wherein the cementitiouscomposition further comprises an aggregate material.
 6. A methodaccording to claim 1, wherein the cementitious composition furthercomprises fibers as reinforcement.
 7. A method according to claim 1,wherein the cementitious composition further comprises water.
 8. Amethod according to claim 7, wherein the cementitious compositioncomprises not more than 50 parts by weight water, based on the weight ofthe cementitious composition when wet.
 9. A method according to claim 1,including, between steps (a) and (b), the step of mixing thecementitious composition with sand to form a cementitious mortarcomposition, prior to being applied to said surface.
 10. A methodaccording to claim 1 consisting of steps (a), (b) and (c).
 11. Adrinking water pipe comprising a hollow metallic conduit and a coatingprovided on at least one of an internal and external surface of theconduit, wherein the coating comprises Portland cement in combinationwith metakaolin, and wherein the coating comprises 5 to 30 parts byweights metakaolin, based on the weight of the coating when dry.
 12. Apipe according to claim 11, wherein the coating comprises not more than10 parts by weight metakaolin.
 13. A pipe according to claim 11, whereinpart of the cement is replaced with a cement replaceable material in anamount of not more than 70 parts by weight.
 14. A pipe according toclaim 13, wherein said cement replacement material is at least one ofground granulated blast furnace slag and pulverized fuel ash.
 15. A pipeaccording to claim 11, wherein the cement is calcium silicate cement.16. A pipe according to claim 11, further comprising at least one ofaggregate and fibers as reinforcement.
 17. A pipe according to claim 16,wherein the coating comprises not more than 50 parts by weight water,based on the weight of the cementitious composition when wet.